Quick viewing(Text Mode)

Heredity Inherited Genes Determine an Organism’S Traits

Heredity Inherited Genes Determine an Organism’S Traits

Heredity Inherited determine an ’s traits.

Section 1 Main Idea Using scientific methods, discovered the basic principles of genetics.

Section 2 Genetics Since Mendel Main Idea It is now known that interactions among , genes, and the environment determine an organism’s traits.

Section 3 Advances in Genetics Main Idea Through , scien- tists can change the DNA of to improve them, increase resistance to insects and , or produce . Why do people look different? People have different skin colors, different kinds of hair, and dif- ferent heights. Knowing how these differences are determined will help you predict when certain traits might appear. This will help you understand what causes hereditary disorders and how these are passed from generation to generation. Science Journal Write three traits that you have and how you would determine how those traits were passed to you.

126 ◆ A Ron Chapple/Getty Images Start-Up Activities

Classify Characteristics As you read this chapter about hered- ity, you can use the following Who around you has dimples? Foldable to help you classify characteristics as inherited or not inherited. You and your best friend enjoy the same sports, like the same food, and even have similar STEP 1 Fold the top of a vertical piece of haircuts. But, there are noticeable differences paper down and the bottom up to between your appearances. Most of these dif- divide the paper into thirds. ferences are controlled by the genes you inher- ited from your . In the following lab, you will observe one of these differences.

STEP 2 Turn the paper horizontally; unfold and label the three columns as shown.

Personal Inherited Not Characteristics Inherited eyes hair dimples

1. Notice the two students in the photo- Read for Main Ideas Before you read the chap- graphs. One student has dimples when ter, list personal characteristics and predict she smiles, and the other student doesn’t which are inherited or not inherited. As you read have dimples. the chapter, check and change your list. 2. Ask your classmates to smile naturally. In your Science Journal, record the name of each classmate and whether each one has dimples. Preview this chapter’s content 3. Think Critically In your Science Journal, and activities at calculate the percentage of students who booka.msscience.com have dimples. Are facial dimples a com- mon feature among your classmates?

A ◆ 127 (bkgd)Ron Chapple/Getty Images, (cl cr)Geoff Butler Learn It! Visualize by forming mental images of the text as you read. Imagine how the text descriptions look, sound, feel, smell, or taste. Look for any pictures or diagrams on the page that may help you add to your understanding.

Practice It! Read the following paragraph. As you read, use the underlined details to form a picture in your mind.

In a Punnett square for predicting one trait, the letters representing the two alleles from one are written along the top of the grid, one letter per section. Those of the second parent are placed down the side of the grid, one letter per section. Each square of the grid is filled in with one donated by each parent. The letters that you use to fill in each of the squares represent the of possible offspring that the parents could produce. —from page 133

Based on the description above, try to visualize a Punnett square. Now look at the Applying Math feature on page 133. • How closely do these Punnett squares match your mental picture? • Reread the passage and look at the picture again. Did your ideas change? • Compare your image with what others in your class visualized.

Apply It! Read the chapter and list three subjects you were able to visualize. Make a rough sketch showing what you visualized.

128 A ◆ A CHAPTER 5 Heredity TheForming main your idea own is often mental theimages first will sentence help you in a paragraph,remember but what not you always. read.

Use this to focus on the main ideas as you read the chapter. Before you read the chapter, respond to the statements below on your worksheet or on a numbered sheet of paper. • Write an A if you agree with the statement. • Write a D if you disagree with the statement.

After you read the chapter, look back to this page to see if you’ve changed your mind about any of the statements. • If any of your answers changed, explain why. • Change any false statements into true statements. • Use your revised statements as a study guide.

Before You Read Statement After You Read A or D A or D

1 The two alleles of a can be the same or different.

2 Alleles are either dominant or recessive.

3 An organism’s determines its .

4 A Punnett square shows the actual genetics of offspring from two parents. Print out a worksheet of this page at 5 Traits are determined by more than one gene. booka.msscience.com 6 Some organisms inherit extra .

7 A pedigrees chart can show the of a trait within a .

8 The female parent determines whether an offspring will be male or female.

9 Genetically engineered organisms can produce medicines.

10 Sex-linked disorders are more common in females than in males.

128 B Genetics Inheriting Traits Do you look more like one parent or ? Do you have your ’s eyes? What about Isabella’s cheekbones? , nose shape, and many other physical features are ■ Explain how traits are inherited. ■ Identify Mendel’s role in the his- some of the traits that are inherited from parents, as Figure 1 tory of genetics. shows. An organism is a collection of traits, all inherited from its ■ Use a Punnett square to predict parents. Heredity (huh REH duh tee) is the passing of traits the results of crosses. from parent to offspring. What controls these traits? ■ Compare and contrast the dif- ference between an individual’s genotype and phenotype. What is genetics? Generally, genes on chromosomes con- trol an organism’s form and function or traits. The different forms of a trait that make up a gene pair are called alleles (uh Heredity and genetics help explain LEELZ). When a pair of chromosomes separates during why people are different. (mi OH sus), alleles for each trait also separate into different sex cells. As a result, every sex has one allele for each trait. In Review Vocabulary Figure 2, the allele in one sex cell controls one form of the trait meiosis: reproductive process that produces four haploid sex for having facial dimples. The allele in the other sex cell controls cells from one diploid cell a different form of the trait—not having dimples. The study of how traits are inherited through the interactions of alleles is the New Vocabulary science of genetics (juh NE tihks). • heredity • Punnett square • allele • genotype • genetics • phenotype • • homozygous • dominant • heterozygous • recessive

Figure 1 Note the strong family resemblance among these four generations.

128 ◆ A CHAPTER 5 Heredity Stewart Cohen/Stone/Getty Images Figure 2 An allele is one form of a gene. Alleles separate into separate sex cells during meiosis. In this example, the alleles that control the trait for dimples include D, the presence of dimples, and d, the absence of dimples.

D D

DD D D d d Female Eggs Meiosis Fertilization

D d dd Dd d d Male Sperm Fertilized eggs

The alleles that control a trait During meiosis, duplicated chro- During fertilization, each parent are located on each duplicated mosomes separate. donates one . This chromosome. results in two alleles for the trait of dimples in the new individual formed. Mendel—The Father of Genetics Did you know that an experiment with pea plants helped scientists understand why your eyes are the color that they are? Gregor Mendel was an Austrian monk who studied mathemat- ics and science but became a gardener in a monastery. His inter- est in plants began as a in his father’s orchard where he could predict the possible types of flowers and fruits that would result from crossbreeding two plants. Curiosity about the con- nection between the color of a pea flower and the type of seed that same plant produced inspired him to begin experimenting with garden peas in 1856. Mendel made careful use of scientific methods, which resulted in the first recorded study of how traits pass from one generation to the next. After eight years, Mendel presented his results with pea plants to scientists. Before Mendel, scientists mostly relied on observation and description, and often studied many traits at one time. Mendel was the first to trace one trait through several generations. He Topic: Genetics was also the first to use the mathematics of probability to Visit booka.msscience.com for Web explain heredity. The use of math in plant science was a new links to information about early concept and not widely accepted then. Mendel’s work was genetics experiments. forgotten for a long time. In 1900, three plant scientists, work- Activity List two other scientists ing separately, reached the same conclusions as Mendel. Each who studied genetics, and what plant scientist had discovered Mendel’s writings while doing his organism they used in their own research. Since then, Mendel has been known as the father research. of genetics.

SECTION 1 Genetics A ◆ 129 Table 1 Traits Compared by Mendel Shape Color Color Shape Plant Position Flower Traits of Seeds of Seeds of Pods of Pods Height of Flowers Color Dominant Round Yellow Green Full Tall At leaf Purple trait junctions

Recessive Wrinkled Green Yellow Flat, short At tips of White trait constricted branches

Genetics in a Garden Each time Mendel studied a trait, he crossed two plants with different expressions of the trait and found that the new plants all looked like one of the two parents. He called these new plants hybrids (HI brudz) because they received different genetic information, or different alleles, for a trait from each parent. The results of these studies made Mendel even more curious about how traits are inherited. Garden peas are easy to breed for pure traits. An organism Comparing that always produces the same traits generation after generation Common Traits is called a purebred. For example, tall plants that always produce Procedure seeds that produce tall plants are purebred for the trait of tall 1. Safely survey as many height. Table 1 shows other pea plant traits that Mendel studied. dogs in your neighborhood as you can for the presence Why might farmers plant purebred crop seeds? of a solid color or spotted coat, short or long hair, and floppy or upright ears. Dominant and Recessive Factors In nature, insects 2. Make a data table that lists randomly pollinate plants as they move from flower to flower. In each of the traits. Record his experiments, Mendel used pollen from the flowers of pure- your data in the data table. bred tall plants to pollinate by hand the flowers of purebred Analysis short plants. This process is called cross-pollination. He found 1. Compare the number of that tall plants crossed with short plants produced seeds that dogs that have one form of produced all tall plants. Whatever caused the plants to be short a trait with those that have had disappeared. Mendel called the tall form the dominant the other form. 2. What can you conclude (DAH muh nunt) factor because it dominated, or covered up, about the variations you the short form. He called the form that seemed to disappear the noticed in recessive (rih SE sihv) factor. Today, these are called dominant the dogs? alleles and recessive alleles. What happened to the recessive form? Figure 3 answers this question.

130 ◆ A CHAPTER 5 Heredity VISUALIZING MENDEL’S EXPERIMENTS

Figure 3

regor Mendel discovered that the experiments he carried out on garden plants provided an understanding of heredity. For eight years he Gcrossed plants that had different characteristics and recorded how those characteristics were passed from generation to generation. One such character- istic, or trait, was the color of pea pods. The results of Mendel’s experiment on pea pod color are shown below.

Parents A One of the so-called “parent plants” in Mendel’s experiment had pods that were green, a dominant trait. The other parent plant had pods that were yellow, a recessive trait. 1st Generation

B Mendel discovered that the two “parents” produced a generation of plants with green pods. The recessive color—yellow—did not appear in any of the pods.

2nd Generation C Next,Mendel collected seeds from the first- generation plants and raised a second generation. He discovered that these second-generation plants produced plants with either green or yellow pods in a ratio of about three plants with green pods for every one plant with yellow pods. The recessive trait had reappeared. This 3:1 ratio proved remarkably consistent in hundreds of similar crosses, allowing Mendel to accurately predict the ratio of pod color in second-generation plants.

SECTION 1 Genetics A ◆ 131

(bkgd)Jane Grushow from Grant Heilman, (others)Special Collections, National Agriculture Library Using Probability to Make Predictions If you and your can’t agree on what movie to see, you could solve the prob- lem by tossing a coin. When you toss a coin, you’re dealing with probabilities. Probability is a branch of mathematics that helps you predict the chance that something will happen. If your sis- ter chooses tails while the coin is in the air, what is the probabil- ity that the coin will land tail-side up? Because a coin has two sides, there are two possible outcomes, heads or tails. Therefore, the probability of tails is one out of two, or 50 percent. Mendel also dealt with probabilities. One of the things that made his predictions accurate was that he worked with large numbers of plants. He studied almost 30,000 pea plants over a period of eight years. By doing so, Mendel increased his chances of seeing a repeatable pattern. Valid scientific conclusions need to be based on results that can be duplicated.

Punnett Squares Suppose you wanted to know what colors of pea plant flowers you would get if you pollinated white flow- ers on one pea plant with pollen from purple flowers on a dif- ferent plant. How could you predict what the offspring would look like without making the cross? A handy tool used to predict results in Mendelian genetics is the Punnett (PUH nut) square. In a Punnett square, letters represent dominant and recessive alle- les. An uppercase letter stands for a dominant allele. A lowercase letter stands for a recessive allele. The letters are a form of code. They show the genotype (JEE nuh tipe), or genetic makeup, of an organism. Once you understand what the letters mean, you Figure 4 This snapdragon’s can tell a lot about the inheritance of a trait in an organism. phenotype is red. The way an organism looks and behaves as a result of its Determine Can you tell what the genotype is its phenotype (FEE nuh tipe), as shown in flower’s genotype for color is? Figure 4. If you have brown hair, then the phenotype for your Explain your answer. hair color is brown.

Alleles Determine Traits Most cells in your body have at least two alleles for every trait. These alleles are located on sim- ilar pairs of chromosomes within the nucleus of cells. An organ- ism with two alleles that are the same is called homozygous (hoh muh ZI gus) for that trait. For Mendel’s peas, this would be written as TT (homozygous for the tall-dominant trait) or tt (homozygous for the short-recessive trait). An organism that has two different alleles is called heterozygous (he tuh roh ZI gus) for that trait. The hybrid plants Mendel produced were all heterozygous for height, Tt.

What is the difference between homozygous and heterozygous organisms?

132 ◆ A CHAPTER 5 Heredity Barry L. Runk From Grant Heilman Making a Punnett Square In a Punnett square for predict- ing one trait, the letters representing the two alleles from one parent are written along the top of the grid, one letter per sec- tion. Those of the second parent are placed down the side of the grid, one letter per section. Each square of the grid is filled in with one allele donated by each parent. The letters that you use to fill in each of the squares represent the genotypes of possible offspring that the parents could produce.

Calculate Percentages

PUNNET SQUARE One dog carries heterozygous, black-fur traits (Bb), and its mate carries homogeneous, blond-fur traits (bb). Use a Punnett square to determine the probability of one of their puppies having black fur.

Solution This is what you know: ● dominant allele is represented by B

● recessive allele is represented by b This is what you need to What is the probability of a puppy’s fur color being black? find out:

This is the procedure ● Complete the Punnett square. Black dog B b you need to use: ● There are two Bb genotypes and four possible b Bb bb outcomes. Blond dog ● %(black fur) ϭ b Bb bb number of ways to get black fur ᎏᎏᎏᎏ Genotypes of offspring: 2Bb, 2bb 2 1 ϭ ᎏᎏ ϭ ᎏᎏ ϭ 50% of offspring: 4 2 2 black, 2 blond 1 Check your answer: ᎏᎏ of 4 is 2, which is the number 2 of black dogs.

Parent (Y y) 1. In peas, the color yellow (Y) is dominant to the color green Y y (y). According to the Punnett square, what is the probability of an offspring being yellow? YY Y Y y Parent 2. What is the probability of an offspring having the yy (Y y) genotype? yY y y y

For more practice, visit booka.msscience.com/ math_practice

SECTION 1 Genetic A ◆ 133 Principles of Heredity Even though Gregor Mendel didn’t know anything about DNA, genes, or chromosomes, he suc- ceeded in beginning to describe and mathematically represent how inherited traits are passed from parents to offspring. He realized that some factor in the pea plant produced certain traits. Mendel also concluded that these factors separated when the pea plant reproduced. Mendel arrived at his conclusions after years of detailed observation, careful analysis, and repeated experi- mentation. Table 2 summarizes Mendel’s principles of heredity.

Table 2 Principles of Heredity 1 Traits are controlled by alleles on chromosomes. 2 An allele’s effect is dominant or recessive. 3 When a pair of chromosomes separates during meiosis, the different alleles for a trait move into separate sex cells.

Summary Self Check Inheriting Traits 1. Contrast Alleles are described as being dominant or Heredity is the passing of traits from parent to recessive. What is the difference between a dominant • offspring. and a recessive allele? Genetics is the study of how traits are inher- 2. Describe how dominant and recessive alleles are repre- • ited through the interactions of alleles. sented in a Punnett square. Mendel—The Father of Genetics 3. Explain the difference between genotype and pheno- type. Give examples. In 1856, Mendel began experimenting with • garden peas, using careful scientific methods. 4. Infer Gregor Mendel, an Austrian monk who lived in the 1800s, is known as the father of genetics. Explain Mendel was the first to trace one trait why Mendel has been given this title. • through several generations. 5. Think Critically If an organism expresses a recessive In 1900, three plant scientists separately phenotype, can you tell the genotype? Explain your • reached the same conclusions as Mendel. answer by giving an example. Genetics in a Garden Hybrids receive different genetic information • for a trait from each parent. Genetics involves dominant and recessive factors. 6. Use Percentages One fruit fly is heterozygous for long • wings, and another fruit fly is homozygous for short • Punnett squares can be used to predict the wings. Long wings are dominant to short wings. Use a results of a cross. Punnett square to find the expected percent of off- Mendel’s conclusions led to the principles of spring with short wings. • heredity.

134 ◆ A CHAPTER 5 Heredity booka.msscience.com/self_check_quiz Richard Hutchings/Photo Researchers Predicting Results Could you predict how many brown rabbits would 5. Without looking, remove one bean from each result from crossing two heterozygous black rab- bag and record the results on your data table. bits? Try this investigation to find out. Brown color Return the beans to their bags. is a recessive trait for hair color in rabbits. 6. Repeat step five 49 more times. 7. Count and record the total numbers for each Real-World Question of the three combinations in your data table. How does chance affect combinations of genes? 8. Compile and record the class totals. Goals ■ Model chance events in heredity. Conclude and Apply ■ Compare and contrast predicted and 1. Name the combination that occurred most actual results. often. Materials 2. Calculate the ratio of red/red to red/white paper bags (2) white beans (100) to white/white. What hair color in rabbits red beans (100) do these combinations represent? 3. Compare your predicted (expected) results Safety Precautions with your observed (actual) results. 4. Hypothesize how you could get predicted WARNING: Do not taste,eat,or drink any mate- results to be closer to actual results. rials used in the lab. Gene Combinations Procedure Rabbits Red/ Red/ White/ 1. Make a Punnett square for a cross between Red White White two heterozygous black rabbits, (Bb ʕ Bb). Your total Do not write in this book. B represents the black allele and b represents Class total the brown allele. 2. Model the above cross by placing 50 red beans and 50 white beans in one paper bag and 50 red beans and 50 white beans in a second bag. Red beans represent black alle- Write a paragraph that clearly describes les and white beans represent brown alleles. your results. Have another student read 3. Label one of the bags Female for the female your paragraph. Ask if he or she could parent. Label the other bag Male for the understand what happened. If not, rewrite male parent. your paragraph and have the other student 4. Use a data table to record the combination read it again. For more help, refer to the each time you remove two beans. Your table Science Skill Handbook. will need to accommodate 50 picks.

LAB A ◆ 135 Genetics Since Mendel Incomplete Not even in science do things remain the same. After Mendel’s work was rediscovered in 1900, scientists repeated his ■ Explain how traits are inherited experiments. For some plants, such as peas, Mendel’s results by incomplete dominance. proved true. However, when different plants were crossed, the ■ Compare multiple alleles and results were sometimes different. One scientist crossed purebred polygenic inheritance, and give red-flowered four-o’ plants with purebred white-flowered examples of each. four-o’clock plants. He expected to get all red flowers, but they ■ Describe two human genetic dis- orders and how they are inherited. were pink. Neither allele for flower color seemed dominant. Had ■ Explain how sex-linked traits are the colors become blended like paint colors? He crossed the passed to offspring. pink-flowered plants with each other, and red, pink, and white flowers were produced. The red and white alleles had not become blended. Instead, when the allele for white flowers and Most of your inherited traits involve the allele for red flowers combined, the result was an intermedi- more complex patterns of inheri- tance than Mendel discovered. ate phenotype—a pink flower. When the offspring of two homozygous parents show an Review Vocabulary intermediate phenotype, this inheritance is called incomplete gene: section of DNA on a chro- dominance. Other examples of incomplete dominance include mosome that contains instruc- the flower color of some plant breeds and the coat color of some tions for making specific proteins horse breeds, as shown in Figure 5. New Vocabulary • incomplete dominance • polygenic inheritance • sex-linked gene

Figure 5 When a chestnut horse is bred with a cremello horse, all offspring will be palomino. The Punnett square shown on the opposite page can be used to predict this result. Explain how the color of the palomino horse shows that the coat color of horses may be inherited by Chestnut horse incomplete dominance. Cremello horse

136 ◆ A CHAPTER 5 Heredity (l)Robert Maier/Animals Animals, (r)Gemma Giannini from Grant Heilman Multiple Alleles Mendel studied traits in peas that were con- trolled by just two alleles. However, many traits are controlled by more than two alleles. A trait that is controlled by more than two alleles is said to be controlled by multiple alleles. Traits con- Topic: Types trolled by multiple alleles produce more than three phenotypes Visit booka.msscience.com for Web of that trait. links to information about the Imagine that only three types of coins are made—nickels, importance of blood types in blood transfusions. dimes, and quarters. If every person can have only two coins, six different combinations are possible. In this problem, the coins Activity Make a chart showing represent alleles of a trait. The sum of each two-coin combina- which blood types can be used for transfusions into people with A, B, tion represents the phenotype. Can you name the six different AB, or O blood phenotypes. phenotypes possible with two coins? in humans is an example of multiple alleles that produce only four phenotypes. The alleles for blood types are called A, B, and O. The O allele is recessive to both the A and B alleles. When a person inherits one A allele and one B allele for blood type, both are expressed—phenotype AB. A person with phenotype A blood has the genetic makeup, or genotype—AA or AO. Someone with phenotype B blood has the genotype BB or BO. Finally, a person with phenotype O blood has the geno- type OO.

What are the six different blood type genotypes?

Punnett square Chestnut horse (CC) C C

C ' C C ' C C ' Cremello horse (C ' C ') C ' C C ' C C '

Genotypes: All C C ' Phenotypes: All palomino horses

Palomino horse

SECTION 2 Genetics Since Mendel A ◆ 137 Raymond Gehman/CORBIS Polygenic Inheritance Eye color is an example of a trait that is produced by a combination of many genes. Polygenic (pah lih JEH nihk) inheritance occurs when a group of gene pairs acts together to Interpreting produce a trait. The effects of many alleles produces a wide vari- Polygenic Inheritance ety of phenotypes. For this reason, it may be hard to classify all Procedure the different shades of eye color. 1. Measure the hand spans of Your height and the color of your eyes and skin are just some your classmates. 2. Using a ruler, measure of the many human traits controlled by polygenic inheritance. It from the tip of the thumb is estimated that three to six gene pairs control your skin color. to the tip of the little finger Even more gene pairs might control the color of your hair and when the hand is stretched eyes. The environment also plays an important role in the out. Read the measurement expression of traits controlled by polygenic inheritance. to the nearest centimeter. Polygenic inheritance is common and includes such traits as 3. Record the name and grain color in wheat and milk production in cows. Egg produc- hand-span measurement of each person in a data tion in chickens is also a polygenic trait. table. Analysis Impact of the Environment Your environment plays a 1. What range of hand spans role in how some of your genes are expressed or whether they did you find? are expressed at all, as shown in Figure 6. Environmental influ- 2. What are the mean, ences can be internal or external. For example, most male median, and mode for your birds are more brightly colored than females. Chemicals in class’s data? their bodies determine whether the gene for brightly colored 3. Are hand spans inherited feathers is expressed. as a simple Mendelian pat- tern or as a polygenic or Although genes determine many of your traits, you might be incomplete dominance able to influence their expression by the decisions you make. pattern? Explain. Some people have genes that make them at risk for developing certain . Whether they get might depend on exter- nal environmental factors. For instance, if some people at risk for skin cancer limit their exposure to the Sun and take care of their skin, they might never develop cancer.

What environmental factors might affect the size of leaves on a tree?

Figure 6 Himalayan rabbits have alleles for dark-colored fur. However, this allele is able to express itself only at lower temper- atures. Only the areas located far- thest from the rabbit’s main body heat (ears, nose, feet, tail) have dark-colored fur.

138 ◆ A CHAPTER 5 Heredity Dan McCoy from Rainbow Human Genes and Sometimes a gene undergoes a change that results in a trait that is expressed differently. Occasionally errors occur in the DNA when it is copied inside of a cell. Such changes and errors Genetic Counselor Testing are called mutations. Not all mutations are harmful. They might for genetic disorders may allow many affected indi- be helpful or have no effect on an organism. viduals to seek treatment Certain chemicals are known to produce mutations in plants and cope with their dis- or animals, including humans. X rays and radioactive substances eases. Genetic counselors are other causes of some mutations. Mutations are changes in are trained to analyze a genes. family’s history to deter- mine a person’s health risk. Research what a genetic Chromosome Disorders In addition to individual muta- counselor does and how to tions, problems can occur if the incorrect number of chromo- become a genetic counselor. somes is inherited. Every organism has a specific number of Record what you learn in chromosomes. However, mistakes in the process of meiosis can your Science Journal. result in a new organism with more or fewer chromosomes than normal. A change in the total number of human chromosomes is usually fatal to the unborn embryo or fetus, or the baby may die soon after birth. Look at the photo of human chromosome 21 in Figure 7. If three copies of this chromosome are in the fertilized human egg, Down results. Individuals with Down syndrome can be short, exhibit disabilities, and have heart problems. Such individuals can lead normal if they have no severe health complications.

Figure 7 Humans usually have 46 chromosomes arranged as 23 pairs. If a person inherits three copies of chromosome 21 instead of the usual two, Down syndrome results. Chris Burke, a well-known actor, has Down syndrome.

SECTION 2 Genetics Since Mendel A ◆ 139 (l)Phil Roach/Ipol, Inc., (r)CNRI/Science Photo Library/Photo Researchers Recessive Genetic Disorders Many human genetic disorders, such as cys- tic fibrosis, are caused by recessive genes. Some recessive genes are the result of a within the gene. Many of these alleles are rare. Such genetic disorders occur when both par- ents have a recessive allele for this disorder. Because the parents are heterozygous, they don’t show any symptoms. However, if each parent passes the recessive allele to the , the child inherits both recessive alleles and will have a recessive .

How is cystic fibrosis inherited? Color-enhanced SEM Magnification:16000ϫ Cystic fibrosis is a homozygous recessive disorder. It is the Figure 8 Sex in many organ- most common genetic disorder that can lead to death among isms is determined by X and Y Caucasian Americans. In most people, a thin fluid is produced chromosomes. that lubricates the lungs and intestinal tract. People with cystic Observe How do the X (left) and Y (right) chromosomes differ from one fibrosis produce thick mucus instead of this thin fluid. The thick another in shape and size? mucus builds up in the lungs and makes it hard to breathe. This buildup often results in repeated bacterial respiratory infections. The thick mucus also reduces or prevents the flow of substances necessary for digesting food. Physical therapy, special diets, and new drug therapies have increased the spans of patients with cystic fibrosis. Gender Determination What determines the gender or sex of an individual? Much information on gender inheritance came from studies of fruit flies. Fruit flies have only four pairs of chromosomes. Because the chromosomes are large and few in number, they are easy to study. Scientists identified one pair that contains genes that determine the sex of the organism. They labeled the pair XX in females and XY in males. use these labels when studying organisms, including humans. You can see human X and Y chromosomes in Figure 8. Each egg produced by a female normally contains one X chro- mosome. Males produce sperm that normally have either an X or a . When a sperm with an fertilizes an egg, the offspring is a female, XX. A male offspring, XY, is the result of a Y-containing sperm fertilizing an egg. What pair of sex chromosomes is in each of your cells? Sometimes chromosomes do not separate during meiosis. When this occurs, an individual can inherit an abnormal number of sex chromosomes.

140 ◆ A CHAPTER 5 Heredity Gopal Murti/PhotoTake, NYC Sex-Linked Disorders A sex-linked gene is an allele on a . Some conditions that result from inheriting a sex-linked gene are called sex-linked disorders. Red-green in humans is a sex-linked disorder because the related genes are on the X chromosome. People who inherit this disorder have diffi- culty seeing the difference between green and red, and some- times, yellow. This condition is a recessive sex-linked disorder. A female is color-blind when each of her X chromosomes has the recessive allele. A male has only one X chromosome and, if it has the recessive allele, he will be color-blind. Dominant sex-linked disorders are rare and result when a person inherits at least one dominant sex-linked allele. Vitamin D-resistant rickets is an X-linked dominant disorder. The kid- neys of an affected person cannot absorb adequate amounts of phosphorus. The person might have low blood-phosphorus lev- els, soft bones, and poor teeth formation. Pedigrees Trace Traits How can you trace a trait through a family? A pedigree is a visual tool for following a trait through generations of a family. Males are represented by squares and females by circles. A com- pletely filled circle or square shows that the trait is seen in that person. Half-colored circles or squares indicate carriers. A car- rier is heterozygous for the trait, and it is not seen. People rep- resented by empty circles or squares do not have the trait and are not carriers. The pedigree in Figure 9 shows how the trait for color blindness is carried through a family.

Pedigree Key Figure 9 The symbols in this pedigree’s key mean the same Normal female Normal male thing on all pedigree charts. The Carrier female Color-blind male grandfather in this family was color-blind and married to a Parents woman who was not a carrier of the color-blind allele. Infer why no women in this family are color-blind.

Children

Grandchildren

SECTION 2 Genetics Since Mendel A ◆ 141 Tim Davis/Photo Researchers Using Pedigrees A pedigree is a useful tool for a . Sometimes a geneticist needs to understand who has had a trait in a family over several generations to deter- mine its pattern of inheritance. A geneticist determines if a trait is recessive, dominant, sex-linked, or has some other pattern of inheritance. When geneticists understand how a trait is inherited, they can predict the Shih Tzu probability that a baby will be born with a specific trait. Pedigrees also are important in breed- ing animals or plants. Because livestock and plant crops are used as sources of food, Figure 10 these organisms are bred to increase their A variety of traits are yield and nutritional content. Breeders of considered when pets and show animals, like the dogs pic- breeding dogs. tured in Figure 10, also examine pedigrees Black Labrador carefully for possible desirable physical and ability traits. Issues concerning health also are considered when researching pedigrees.

Summary Self Check Incomplete Dominance 1. Compare how inheritance by multiple alleles and poly- Incomplete dominance is when a dominant genic inheritance are similar. • and recessive allele for a trait show an inter- 2. Explain why a trait inherited by incomplete dominance mediate phenotype. is not a blend of two alleles. Many traits are controlled by more than two 3. Discuss Choose two genetic disorders and discuss how • alleles. they are inherited. A wide variety of phenotypes is produced by 4. Apply Using a Punnett square, explain why males are • polygenic inheritance. affected more often than females by sex-linked genetic Human Genes and Mutations disorders. Errors can occur when DNA is copied. 5. Think Critically Why wouldn’t a horse breeder mate • male and female palominos to get palomino colts? Mistakes in meiosis can result in an unequal • number of chromosomes in sex cells. Recessive genes control many human genetic • disorders. 6. Predict A man with blood type B marries a woman with blood type A. Their first child has blood type O. Sex Determination Use a Punnett square to predict what other blood An allele inherited on a sex chromosome is types are possible for their offspring. • called a sex-linked gene. 7. Communicate In your Science Journal, explain why Pedigrees are visual tools to trace a trait offspring may or may not resemble either parent. • through generations of a family.

142 ◆ A CHAPTER 5 Heredity booka.msscience.com/self_check_quiz (t)Renee Stockdale/Animals Animals, (b)Alan & Sandy Carey/Photo Researchers Advances in Genetics Why is genetics important? If Mendel were to pick up a daily newspaper in any country today, he’d probably be surprised. News articles about develop- ments in genetic research appear almost daily. The term gene has ■ Evaluate the importance of become a common word. The principles of heredity are being advances in genetics. used to change the world. ■ Sequence the steps in mak- ing genetically engineered Genetic Engineering organisms. You might recall that chromosomes are made of DNA and are in the nucleus of a cell. Sections of DNA in chromosomes that Advances in genetics can affect your direct cell activities are called genes. Through genetic engineering, health, the foods that you eat, and scientists are experimenting with biological and chemical meth- your environment. ods to change the arrangement of DNA that makes up a gene. Genetic engineering already is used to help produce large vol- Review Vocabulary DNA: deoxyribonucleic acid; the umes of . Genes also can be inserted into cells to genetic material of all organisms change how those cells perform their normal functions, as shown in Figure 11. Other research is being done to find new New Vocabulary ways to improve crop production and quality, including the • genetic engineering development of plants that are resistant to .

Figure 11 DNA from one organism is placed into another Bacterial cell . This method is used to produce human insulin, human growth hormone, and other chemicals by bacteria.

More genetically engineered cells making insulin DNA loop DNA chromosome

Insulin DNA loop produced cut and removed using chemicals; Reproduction cut section is lost

Insulin gene from human chromosome

Another bacterial cell Recombinant takes up recombinant DNA DNA formed

SECTION 3 Advances in Genetics A ◆ 143 Recombinant DNA Making recombinant DNA is one method of genetic engineering. Recombinant DNA is made by inserting a useful segment of DNA from one organism into a Genetically Engineered bacterium, as illustrated in Figure 11. Large quantities of human Crops Crop plants are now being genetically insulin are made by some genetically engineered organisms. engineered to produce People with Type 1 need this insulin because their chemicals that kill specif- pancreases produce too little or no insulin. Other uses include ic pests that feed on the production of growth hormone to treat dwarfism and them. Some of the pollen chemicals to treat cancer. from pesticide-resistant canola crops is capable of Gene Transfer spreading up to 8 km Another application of genetic-engineering is from the plant, while gene transfer. A goal of this experimental procedure is to replace corn and potato pollen abnormal genetic material with normal genetic material. First, can spread up to 1 km. normal DNA or RNA is placed in a virus. Then the virus delivers What might be the the normal DNA or RNA to target cells, as shown in Figure 12. effects of pollen landing Gene transfer, also known as , might help correct on other plants? genetic disorders such as cystic fibrosis. It also is being studied as a possible treatment for cancer, heart disease, and certain infectious diseases.

Normal gene +

Cold virus

Figure 12 Gene transfer involves placing normal genetic material into a cell with abnormal genetic material. When the normal genetic material begins to function, Normal gene inserted into the abnormal condition is corrected. cold virus

Nucleus Nasal cell Normal gene

Lung

144 ◆ A CHAPTER 5 Heredity Genetically Engineered Plants For thousands of years people have improved the plants they use for food and clothing even without the knowledge of genotypes. Until recently, these improvements were the results of selecting plants with the most desired traits to breed for the next generation. This process is called selective breeding. Recent advances in genetics have not replaced selective breeding. Although a plant can be bred for a particular phenotype, the genotype and pedigree of the plants also are considered. Genetic engineering can produce improvements in crop plants, such as corn, wheat, and rice. One type of genetic engi- neering involves finding the genes that produce desired traits in one plant and then inserting those genes into a different plant. Scientists recently have made genetically engineered tomatoes with a gene that allows tomatoes to be picked green and trans- Figure 13 Genetically engi- ported great distances before they ripen completely. Ripe, firm neered produce is sometimes tomatoes are then available in the local market. In the future, labeled. This allows consumers additional food crops may be genetically engineered so that they to make informed choices about are not desirable food for insects. their foods. What other types of traits would be considered desirable in plants? Because some people might prefer foods that are not changed genetically, some stores label such produce, as shown in Figure 13. The long-term effects of consuming genetically engi- neered plants are unknown.

Summary Self Check Why is genetics important? 1. Apply Give examples of areas in which advances in Developments in genetic research appear in genetics are important. • newspapers almost daily. 2. Compare and contrast the technologies of using The world is being changed by the principles recombinant DNA and gene therapy. • of heredity. 3. Infer What are some benefits of genetically engi- neered crops? Genetic Engineering 4. Describe how selective breeding differs from genetic • Scientists work with biological and chemical engineering. methods to change the arrangement of DNA 5. Think Critically Why might some people be opposed to that makes up a gene. genetically engineered plants? One method of genetic engineering is making • recombinant DNA. • A normal allele is replaced in a virus and then 6. Concept Map Make an events-chain concept map of delivers the normal allele when it infects its the steps used in making recombinant DNA. target cell.

booka.msscience.com/self_check_quiz SECTION 3 Advances in Genetics A ◆ 145 Tom Meyers/Photo Researchers Design Your Own Tests for Culur Blindness

Goals Real-World Question ■ Design an experiment What colors do color-blind people see? That that tests for a specific depends on the type of color blindness that they type of color blindness inherit. The most common type is red-green color in males and females. blindness in which people have difficulty seeing ■ Calculate the per- any difference between red and green. People centage of males with another inherited type cannot distinguish and females with between blue and yellow. In rare instances, a the disorder. person can inherit a type of color blindness where the only colors seen are shades of gray. Possible Materials What percentages of males and females in your white paper or poster school are color-blind? board colored markers: red, orange, yellow, bright green, dark green, blue Form a Hypothesis *computer and color printer Based on your reading and your own experiences, form a hypothesis *Alternate materials about how common color blindness is among males and females.

Test Your Hypothesis Make a Plan 1. Decide what type of color blindness you will test for—the common green-red color blindness or the more rare green-blue color blindness. 2. List the materials you will need and describe how you will create test pictures. Tests for color blindness use many circles of red, orange, and yellow as a background, with circles of dark and light green to make a picture or number. List the steps you will take to test your hypothesis. 3. Prepare a data table in your Science Journal to record your test results.

146 ◆ A CHAPTER 5 Heredity (t)Runk/Schoenberger from Grant Heilman, (b)Mark Burnett 4. Examine your experiment to make sure all steps are in logical order. 5. Identify which pictures you will use as a control and which pictures you will use as variables. Follow Your Plan 1. Make sure your teacher approves your plan before you start. 2. Draw the pictures that you will use to test for color blindness. 3. Carry out your experiment as planned and record your results in your data table.

Analyze Your Data 1. Calculate the percentage of males and females that tested positive for color blindness. 2. Compare the frequency of color blindness in males with the frequency of color blindness in females.

Conclude and Apply 1. Explain whether or not the results supported your hypothesis. 2. Explain why color blindness is called a sex- linked disorder. 3. Infer how common the color-blind disorder is in the general population. 4. Predict your results if you were to test a larger number of people.

Using a word processor, write a short article for the advice column of a fashion magazine about how a color-blind person can avoid wearing outfits with clashing colors. For more help, refer to the Science Skill Handbook.

LAB A ◆ 147 Richard Hutchings The Human Did you know... Centriole . . . The biggest advance in genetics in years took place in February 2001. Scientists successfully mapped the human genome. There Nucleus are 30,000 to 40,000 genes in the human Endoplasmic genome. Genes are in the nucleus of each of reticulum the several trillion cells in your body. Cytoplasm

. . . The strands of DNA in the human genome, if unwound and connected end to end, would be more than 1.5 m long—but only about Human DNA 130 trillionths of a centimeter hair wide. Even an average human hair is as much as 200,000 times wider than that.

. . . It would take about nine and one-half years to read aloud without stopping the 3 billion bits of instructions (called base pairs) in your genome.

If one million base pairs of DNA take up 1 megabyte of storage space on a computer, how many gigabytes (1,024 megabytes) would the whole genome fill? Find Out About It Human genome scientists hope to identify the location of disease-causing genes. Visit booka.msscience.com/science_stats to research a genetic disease and share your results with your class.

148 ◆ A CHAPTER 5 Heredity KS Studios Genetics 3. Some disorders are the results of inheritance and can be harmful and even deadly. 1. Genetics is the study of how traits are inherited. Gregor Mendel determined the 4. Pedigree charts help reveal patterns of the basic laws of genetics. inheritance of a trait in a family. Pedigrees show that sex-linked traits are expressed 2. Traits are controlled by alleles on more often in males than in females. chromosomes. 3. Some alleles can be dominant or recessive. Advances in Genetics 4. When a pair of chromosomes separates during meiosis, the different alleles move 1. Genetic engineering uses biological and into separate sex cells. Mendel found that chemical methods to change genes. he could predict the outcome of genetic 2. Recombinant DNA is one method of crosses. genetic engineering to make useful chemi- cals, including hormones. Genetics Since Mendel 3. Gene transfer shows promise for correcting many human genetic disorders, cancer, and 1. Inheritance patterns studied since Mendel other diseases. include incomplete dominance, multiple 4. Breakthroughs in the field of genetic engi- alleles, and polygenic inheritance. neering are allowing scientists to do many 2. These inheritance patterns allow a variety things, such as producing plants that are of phenotypes to be produced. resistant to disease.

Examine the following pedigree for diabetes and explain the inheritance pattern.

Key: = Affected male = Unaffected male = Affected female = Unaffected female

booka.msscience.com/interactive_tutor CHAPTER STUDY GUIDE A ◆ 149 allele p. 128 hybrid p. 130 Choose the word or phrase that best answers the dominant p. 130 incomplete question. genetic engineering p. 143 dominance p. 136 genetics p. 128 phenotype p. 132 11. Which describes the allele that causes genotype p. 132 polygenic inheritance p. 138 color blindness? heredity p. 128 Punnett square p. 132 A) dominant heterozygous p. 132 recessive p. 130 B) carried on the Y chromosome homozygous p. 132 sex-linked gene p. 141 C) carried on the X chromosome D) present only in males Fill in the blanks with the correct word. 12. What is it called when the presence of two Alternate forms of a gene are called 1. different alleles results in an intermediate ______. phenotype? A) incomplete dominance 2. The outward appearance of a trait B) polygenic inheritance is a(n) ______. C) multiple alleles 3. Human height, eye color, and skin color D) sex-linked genes are all traits controlled by ______. 13. What separates during meiosis? A) proteins C) alleles 4. An allele that produces a trait in the het- B) phenotypes D) pedigrees erozygous condition is ______. 14. What controls traits in organisms? 5. ______is the science that deals with A) cell membrane C) genes the study of heredity. B) cell wall D) Punnett squares 6. The actual combination of alleles of an 15. What term describes the inheritance of organism is its ______. cystic fibrosis? A) polygenic inheritance 7. ______is moving fragments of DNA B) multiple alleles from one organism and inserting them C) incomplete dominance into another organism. D) recessive genes

8. A(n) ______is a helpful device for 16. What phenotype will the offspring repre- predicting the probabilities of possible sented in the Punnett square have? genotypes. A) all recessive B) all dominant F f 9. ______is the passing of traits from C) half recessive, parents to offspring. half dominant FF F F f D) Each will have 10. Red-green color blindness is a human a different genetic disorder caused by a(n) phenotype. FF F F f ______.

150 ◆ A CHAPTER REVIEW booka.msscience.com/vocabulary_puzzlemaker 17. Explain the relationship between DNA, 24. Newspaper Article Write a newspaper genes, alleles, and chromosomes. article to announce a new, genetically engineered plant. Include the method of 18. Classify these inheritance patterns: developing the plant, the characteristic a. many different phenotypes produced by changed, and the terms that you would one pair of alleles expect to see. Read your article to b. many phenotypes produced by more the class. than one pair of alleles; two phenotypes from two alleles; three phenotypes from 25. Predict In humans, the widow’s peak allele two alleles. is dominant, and the straight hairline allele is recessive. Predict how both par- Use the illustration below to answer question 19. ents with widow’s peaks could have a child without a widow’s peak hairline.

T t T t 26. Use a word processor or program to write pre- dictions about how advances in genetics might affect your life in the next ten years.

T t T t

27. Human Genome If you wrote the genetic infor- mation for each gene in the human genome on 19. Interpret Scientific Illustrations What were the a separate sheet of 0.2-mm-thick paper and genotypes of the parents that produced stacked the sheets, how tall would the stack be? the Punnett Square shown above? Use the table below to answer question 28. 20. Explain why two rabbits with the same Genome Sizes of Various Organisms genes might not be colored the same if one is raised in northern Maine and one is 80 raised in southern Texas. 60

21. Apply Can a person with a genetic disorder 40 that has been corrected by gene transfer pass the corrected condition to his or her 20 children? Explain. 0 Human Mouse Fruit fly Fungus E. coli 22. Predict Two organisms were found bacteria to have different genotypes but the same Number of genes (thousands) Organism phenotype. Predict what these phenotypes might be. Explain. 28. Genes Consult the graph above. How many more genes are in the human genome than the 23. Compare and contrast genome of the fruit fly? with incomplete dominance.

booka.msscience.com/chapter_review CHAPTER REVIEW A ◆ 151 Record your answers on the answer sheet 5. What is the genotype of the ? provided by your teacher or on a sheet of paper. A. DD C. dd 1. Heredity includes all of the following except B. Dd D. D A. traits. C. nutrients. 6. The genotype of the unaffected children is B. chromosomes. D. phenotype. A. DD. C. dd. B. Dd. D. D. 2. What is a mutation? A. A change in a gene which is harmful, 7. Manipulating the arrangement of DNA that beneficial, or has no effect at all. makes up a gene is called B. A change in a gene which is only A. genetic engineering. beneficial. B. chromosomal migration. C. A change in a gene which is only harmful. C. viral reproduction. D. No change in a gene. D. cross breeding. Use the Punnett square below to answer question 8. 3. Sex of the offspring is determined by A. only the mother, because she has two A O X chromosomes. B. only the father, because he has one X A AA AO and one Y chromosome. C. an X chromosome from the mother and either an X or Y chromosome from the B AB BO father. D. mutations. 8. How many phenotypes would result from Use the pedigree below to answer questions 4–6. the following Punnett square? A. 1 C. 3 Affected female B. 2 D. 4 Normal female Normal male 9. Down syndrome is an example of Affected male A. incomplete dominance. B. genetic engineering. C. a chromosome disorder. D. a sex linked disorder.

Huntington disease has a dominant (DD or Complete Charts Write directly on complex charts such as a Dd) inheritance pattern. Punnett square. 4. What is the genotype of the father? Question 10 Draw a Punnett square to answer all parts of the A. DD C. dd question. B. Dd D. D

152 ◆ A STANDARDIZED TEST PRACTICE Record your answers on the answer sheet Record your answers on a sheet of paper. provided by your teacher or on a sheet of paper. 14. helps individuals deter- Use the table below to answer questions 10–11. mine the genetic risk or probability a disor- der will be passed to offspring. Why would Some Traits Compared by Mendel a pedigree be a very important tool for the TraitsShape Shape Flower counselors? Which patterns of inheritance of Seeds of Pods Color (dominant, recessive, x-linked) would be the easiest to detect?

Dominant 15. Explain the process of gene transfer. What Trait types of disorders would this be best suited? How might gene transfer help Round Full Purple patients with cystic fibrosis? Refer to the figure below to answer question 16. Recessive Trait Flat, Wrinkledconstricted White

10. Create a Punnett square using the Shape of Pods trait crossing heterozygous parents. What percentage of the offspring will be heterozygous? What percentage of the off- spring will be homozygous? What percent- 16. What is the disorder associated with the age of the offspring will have the same karyotype shown above? How does this phenotype as the parents? condition occur? What are the characteris- 11. Gregor Mendel studied traits in pea plants tics of someone with this disorder? that were controlled by single genes. 17. Explain why the parents of someone with Explain what would have happened if the cystic fibrosis do not show any symptoms. alleles for flower color were an example of How are the alleles for cystic fibrosis incomplete dominance. What phenotypes passed from parents to offspring? would he have observed? 18. What is recombinant DNA and how is it 12. Why are heterozygous individuals called used to help someone with Type I diabetes? carriers for non-sex-linked and X-linked recessive patterns of inheritance? 19. If each kernel on an ear of corn represents a separate genetic cross, would corn be a 13. How many alleles does a body cell have for good plant to use to study genetics? Why each trait? What happens to the alleles or why not? What process could be used during meiosis? to control pollination?

booka.msscience.com/standardized_test STANDARDIZED TEST PRACTICE A ◆ 153 CNRI/Science Photo Library/Photo Researchers

Top View